Eggs generally provide most of the water in a cookie mix, as well as proteins that help bind the flour particles together and coagulate during baking to add solidity. The fat and emulsifiers in the yolk enrich and moisten. The higher the proportion of whole eggs or yolks in a recipe, the more cake-like the texture.
From the book On Food and Cooking (2nd edition) by Harold McGee. © 2004 Harold McGee.
By permission of Scribner, a division of Simon & Schuster, Inc.